1. Field of the Invention
The present invention relates to the area of integrated circuit, and more particularly to device and method for controlling an analog signal via a single connection (e.g., a connector or a pin).
2. Description of Related Art
A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This effect is a form of electroluminescence. An LED is a small area source with extra optics added to the chip that makes it emit a certain radiation pattern. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible or near-ultraviolet.
LEDs are commonly used as a lighting source in applications. It is commonly known that a current going through an LED controls the brightness of the emitting light from the LED. However, unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased, thus emitting light. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, thus no light is emitted.
Because the voltage versus current characteristics of an LED are much like any diode (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current, resulting in brightness variations of the LED. Changes in brightness, if not so designed, are not desirable. Accordingly, there is a need for control circuit that is able to regulate a voltage applied across an LED to keep a constant current flowing an LED.
Most of the prior art methods use a current source and an external resistor to modulate the value of a current going through an LED. It has been noticed that the external resistor may introduce various undesirable side-effects. Therefore, there is a need for techniques for generating a precisely controlled current to be applied to an LED without an external element.